The present invention relates to an apparatus and method for scrubbing flue gases to remove oxides of nitrogen and sulfur and, in particular, the present invention relates to an apparatus and method for regenerating the gas scrubbing solution.
In recent years, there has been a growing concern over the problem of air pollution. In some industrialized urban areas, this problem has become acute. There are a variety of sources of air pollution such as the internal combustion engine, fossil fuel power generating plants and metallurgical and chemical plants.
It is well known that the flue gases from the combustion of fossil fuels, such as oil and coal, contain sulfur dioxide and nitrogen oxides. These pollutants occur from the combustion of sulfur and nitrogen compounds in the fuels and, in the case of nitrogen oxides, from the high temperature reaction of nitrogen and oxygen gases at the point of combustion. Nitrogen oxides contribute to air pollution by the formation of chemical smog and sulfur dioxide creates potential health hazards.
One prior solution to this problem is to utilize low sulfur and low nitrogen fuels in fossil fuel burning plants. However, with more stringent regulations regarding flue gas emissions, it has become increasingly important to limit the quantities of these pollutants released into the atmosphere.
Another prior solution to this problem is to wash the flue gas with various scrubbing solution before releasing the gas into the atmosphere. Conventional wet scrubbers typically employ an enclosure wherein the flue gas is washed with the scrubbing solution to remove the pollutants. One prior scrubbing solution comprised an aqueous alkali solution. Although this solution removed sulfur dioxide and nitrogen dioxide from the flue gas, the solution was ineffective in removing nitric oxide from the flue gas. Another prior scrubbing solution comprised an alkaline solution of sulfite or thiosulfite. Although this solution removed oxides of sulfur and nitrogen from the flue gas, removal of the oxides required that the solution be in contact with the flue gas a substantial length of time.
Another prior scrubbing solution comprised an aqueous solution of ferrous chelates. This solution was effective in removing oxides of nitrogen from flue gas. The scrubbing process also resulted in the conversion of sulfur dioxide to sulfate ions and the oxidation of the ferrous chelates to ferric chelates. Since the ferric chelate was ineffective in removing nitrogen oxide from flue gas, it was necessary to regenerate the scrubbing solution by reducing the ferric chelate back to to ferrous chelate by suitable means such as electrolytic reduction before it could be reused in the scrubber. Unfortunately, with continued use, there was an increase in the concentration of sulfate ions in the scrubbing solution. Eventually, the increase in concentration of sulfate ions resulted in the precipitation of a sulfate salt in the scrubber. Prior methods of removing sulfate ions from solution, involving side stream chemical and thermal treatment. However, these methods of treatment are complex and require a large number of process steps. Therefore, an improved process for the removal of oxides of nitrogen and sulfur from flue gases is still desired.